These are optical devices of very small size and very low consumption, particularly sought-after for applications in the field of quantum communications (cryptography, computation, etc.) or for so-called “extreme” integration.
Generally, the photonic crystals are structures with a dielectric index that varies periodically in line with the wavelength, in one or more directions in space. FIG. 1 illustrates the intensity of the electrical field of an electromagnetic wave being propagated in this type of structure and shows the dispersion pattern of a structure of period a in the first Brillouin area; the wave vector k lies within the interval 0<k<π/a. It is known how to artificially structure materials, for example semiconductors, to exploit the diffraction effects and by the same token making it possible to create passive and active optical functions necessary, for example, for optical fiber-based telecommunication networks.
Moreover, one of the major attractions of these structures lies in the controlled insertion of defects within the crystal. These defects can generate states at the prohibited band frequencies of the crystal and thus enable an electromagnetic field that can propagate these frequencies. Control of the propagation of light within the crystal and in step with the wavelength can then be envisaged via these defects. The use of these structures thus opens the way to the miniaturization of integrated optical components.
Compared to the three-dimensional crystalline structures, it has been shown that a two-dimensional structure could be particularly interesting. In this case, crystals are produced in a thin semiconductive guiding layer which provides for better control and a technology that is easier to implement and that is compatible with conventional microelectronics technologies.
A very thin layer is isolated, thus constituting a membrane that can typically have a thickness h of the order of 150 nanometers to 300 nanometers for the applications targeted on the spectral domain between approximately 1 micron and 1.6 microns. By a simple scale law, this thickness is adjusted to extend the application to other spectral domains. The law is as follows: h is between 0.1 and 0.3 times the wavelength. The material used can typically be silicon or a semiconductor material based on elements from columns III and V of Mendeleyev's Periodic Table (semiconductors “III-V”, for example GaAs, AlGaAs, GaInP, InP, AlGaAsP, etc.).
Materials that can also be envisaged are semiconductors from the family II-VI (for example ZnO) as well as SiN.
A waveguide is created within these membranes that has a strong optical index variation as illustrated in FIG. 2. It is notably possible to adjust the speed of propagation of the waves and the dispersion of the guided modes by varying the size of the patterns.
In addition, these devices are extremely compact and can be easily integrated. This means a low consumption, and a very reduced weight and volume, which makes them very attractive for embedded applications. However, the connection with a fiber is a difficult point and there is no current simple and inexpensive solution with which to address this major problem.
In practice, an adaptation must be made between the guided mode of an optical fiber, the size of which is typically of the order of a few hundreds of microns squared (typically of the order of 10 μm in diameter), and that of a waveguide produced Within a photonic crystal that is capable of containing modes in sections of approximately 0.2*0.3 μm2.
Notably proposed by IBM researchers Sharee J. McNab, Nikolaj Moll and Yurii A. Vlasov, in “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides”, OPTICS EXPRESS 2927, 3 Nov. 2003/vol. 11, No. 22, is a technology defining an element adjacent to the photonic crystal and of variable section.
This element made of silicon Si is produced on the surface of a layer of oxide SiO2 and embedded in a polymer referenced Poly. It is thus possible to use it as an adapter. FIGS. 3a and 3b illustrate this adaptive element and FIG. 4 represents the solution recommended in this article which consists in making an adaptation in three stages between the guided modes of the fibers F and those derived from the photonic crystal PhC.
According to this technology, a first element A1 couples the mode of the photonic crystal to that of a suspended ribbon guide. A transition is then made to a ribbon guide resting on the low-index sacrificial layer. Finally, a last element A2 adapts the optical mode of the latter to that of the optical fiber F.
The drawbacks of such a solution lie notably in:                the complexity of design (two mode adapters),        the complexity of production (addition or a plurality of technological steps and inclusion of an additional material: polymer),        the spurious reflections that can occur at the transition between the suspended ribbon and the ribbon resting on the dielectric (in particular, if the latter is not low index),        the spurious reflections on the substrate that can occur and are due to the divergence, however weak, of the beam,        the cleaving accuracy needed to avoid the above reflections,        the proximity effects during the electronic lithography provoked by the change of geometry.        
Moreover, this solution can be produced on an SiO2 substrate, but is difficult to apply to other materials because the portion of the guide in ribbon form must be maintained by a low-index substrate.
The adaptive element on the surface of the layer of SiO2 is then in a heterogeneous environment. It rests on a substrate whereas its top surface is not in contact with the same type of material. This property creates a dissymmetry at the level of the containment of the optical modes.
Also proposed by a Japanese university team—N. Ikeda, H. Kawashima, Y. Sugimoto, T. Hasama, K. Asakawa, H. Ishikawa, in “Coupling characteristic of micro planar lens for 2D photonic crystal waveguides”, Conference Proceedings of IPRM (International Conference on Indium Phosphide and Related Materials) May 2007, is a technology defining a planar lens placed at the termination of a waveguide made of photonic crystal. This element, produced in GaAlAs, is used to reduce the coupling losses and the alignment tolerances while reducing the spurious Fabry-Perot reflections. The main problem with this solution lies in the fact that the radiated beam is not circulated because of the geometrical factors of the microlens; this techniques does not make it possible to reduce the divergence in the vertical plane.